the bubbles are normally down to bad prep work.
as for sanding it down yourself, good money saving tip but some bodyshops wont touch it i would be a bit weary painting it, if there was any defects after paint it could be down to numerous things.
if prepped and painted in the bodyshop you can take it back if any defects came through.
you can get the 200 quid self prep jobs but IMO stay clear from them, for a decent job the materials would cost that!